Imaging polarimetry of cometary dust: different comets and phase angles

Abstract

Polarimetric observations of the light scattered by dust in cometary comae have been carried out at Pic-du-Midi Observatory and at Haute-Provence Observatory (France) between 1990 and 2001. The whole coma polarization can easily be inferred from integration of the fluxes on the aperture. Maps are obtained for seven comets by CCD imaging polarimetry, which allow to observe three main regions in a cometary coma: the background coma, a circumnuclear halo and bright structures (jets or arcs) which correspond to different values of the polarization. The polarization maps of short period comets are compared to the maps obtained for comet Hale-Bopp, for phase angles smaller than 50° ( α smaller than 15° with negative polarization, α near the inversion angle of approximately 20°, α between 30° and 50° on the linear part of the phase curve). At α≅60° the polarization map of comet C/2000 WM1 is presented. Between 109° and 121° the evolution of brightness images and polarization maps of comet C/1999 S4 is followed during the complete disruption of its nucleus. The polarimetric phase curve for the circumnuclear halo presents highly negative values for α<20°. On the opposite for the same phase angles range, the polarization in jets is positive for the four studied comets. The differences between the regions are discussed in terms of physical properties of the particles (size distribution, packing density, mixtures of materials) by comparison to other observational techniques and laboratory measurements.